The ability to reliably detect a specific compound even at low concentrations is a continuing challenge, and there is the on-going need to provide new methodologies and devices to do this. The detection of explosives and explosives-related compounds that are commonly used in terrorist attacks and war zones is a particularly pressing need. Although there is often talk of dirty bombs, biological and nuclear terrorism, and improvised explosive devices (IEDs) the majority of weapons still use nitroaromatic and nitroaliphatic materials either as the explosive, detonator, accelerant or taggant. There remains therefore a clear need to be able to detect these materials with suitably high sensitivity and selectivity.
Many commercially available sensors rely on detection of secondary species that are produced when a target analyte (target molecule) is present in an environment. For example, in such sensors the target analyte may be decomposed by chemical reaction to produce another molecule that is itself then detected. This approach to detection is indirect in nature and while being sensitive can be somewhat inefficient due to the time taken from sampling to detection. Other detection methods require direct contact sampling. The disadvantage of these latter methods is that they are reliant on a specific area of a substrate where the explosive analyte is present to be sampled, for example, swabbed.
An approach for direct detection of a target analyte relies on the use of luminescent compounds. When some compounds are exposed to light of a certain wavelength, they absorb the light (photoexcitation) and emit light of a different wavelength (luminescence, which can either be fluorescence or phosphorescence). This emitted light can be measured/detected. However, certain analyte molecules may also interact with the (excited) luminescent compound to cause an increase or decrease in the intensity of the emitted light. This change can also be detected and, as such, can be used to indicate the presence of the analyte molecules. Sensors embodying the luminescence quenching approach are commercially available. The sensing materials are comprised of thin films containing luminescent conjugated polymers. These sensors can demonstrate high sensitivity but there is scope for improvement in terms of selectivity, especially when the intention is to detect an explosives or explosive-related materials. False positives can occur since every-day products, such as cosmetics, coffee and solvents, can illicit the same qualitative decrease in luminescent intensity as explosives and explosive-related materials. This is a significant limitation on the usefulness of these existing conjugated polymer sensors.
Against this background it would be desirable to provide a sensor technology that does not suffer these drawbacks. It would be particularly desirable to provide a sensor technology that shows high sensitivity to key analytes, such as explosives and explosives-related materials, and that allows the presence of such key analytes to be identified by a characteristic response that per se is specific and selective for those analytes. In this way the presence of every-day chemicals would give a different response to that of the desired explosives and explosives-related materials.